As mobile devices such as portable electronic devices have become widely used, flash memories have been widely used as large-capacitance inexpensive nonvolatile memories that can hold stored data even when power is off. However, in recent years, the limits in miniaturization of flash memories have come into view, and nonvolatile memories such as a magnetic random access memory (MRAM), a phase change random access memory (PCRAM), a conductive bridging random access memory (CBRAM), and a resistance random access memory (ReRAM) have been developed actively.
For example, when ReRAMs are used, a memory cell array can be configured only by stacking variable resistive elements and rectifying elements, which constitute memory cells, at intersections between word lines and bit lines. This allows the construction of a high-density memory system. Further, when memory cell arrays having such a structure are used, memory cell arrays can be stacked in layers into a three-dimensional structure, thus allowing a higher density.
Generally, for ReRAMs, a metal oxide is used as a variable resistive element, and a PIN diode or the like is used as a rectifying element. In order to achieve a higher density, it is necessary to make a memory cell itself smaller.
As one approach to make a memory cell itself smaller, there is a possible method of using organic molecules for a variable resistive element and a rectifying element. According to this method, an organic molecule itself is small in size, and thus allows an element to be reduced in size.
As for a rectifying element of organic molecules, there is a report on rectification by an element with a molecule D-σ-A, in which insulating covalent bonds bridge an electron-donating group (donor group) D with a small ionization potential and an electron-accepting group (acceptor group) A with a high electron affinity, sandwiched between electrodes. Further, there is a report on rectification by an element with a molecule T-D-π-A, in which a bridging portion is replaced with a short π electron and an insulating alkyl chain T (tail) is bonded to D, sandwiched between electrodes.
A donor group D tends to release an electron, and thus tends to accept a hole but does not accept an electron. Thus, a hole flows from an electrode to D, but an electron does not. On the other hand, an acceptor group A tends to accept an electron but does not tend to release an electron, and does not constitute a hole. Thus, an electron flows from an electrode to A, but a hole does not. Thus, a current flows only in a direction from D to A, developing rectification.
On the other hand, for a variable resistive element of organic molecules, there is a report that a self-assembled monolayer of molecules each having an acceptor nitro group and a donor amino group on a phenylene ethynylene skeleton exhibits a negative differential resistance, and develops a memory property that the ON/OFF ratio (the ratio of a current flowing when the same voltage is applied before switching to that after switching) is 1000 or greater.
As described above, there are some examples of reports on an organic molecule to which a donor or acceptor substituent is attached develops rectification and a memory effect. However, there are points to be kept in mind to design a memory cell.
In a ReRAM, a variable resistive element and a rectifying element are connected in series because of its cell structure. Therefore, when the value of a current flowing through a memory cell is low while a variable resistive element is in a low resistance state, a share of voltage applied to the variable resistance element becomes small, thus resulting in an increase in operating voltage. Therefore, the resistance value of the rectifying element in a forward bias direction needs to be sufficiently smaller than the resistance value of the variable resistive element in the low resistance state.
Further, in order to increase the reliability of a cell when reading a signal, it is more preferable for a variable resistive element to have a larger difference between a resistance value in a low resistance state and that in a high resistance state. In other words, it is preferable that the ON/OFF ratio of a variable resistive element be large.
It is not easy to design a molecular device, taking the above points into account. Further, it is difficult to design a molecular device so that a single organic molecule is provided with both a rectifying property and a memory property, taking the above points into account. Thus, there is a room for improvement in the design of an organic molecule for use in a memory cell.